During centrifugation, air friction and electrical power loss produce heat in a centrifuge bowl during rotation of the centrifuge rotor. Since the centrifuge bowl is closed with a cover to prevent material to be centrifuged from escaping, this heat input cannot readily be dissipated and thus leads to an increase in temperature of a material to be centrifuged.
This temperature increase is, however, undesirable since it may lead to destruction and/or uselessness of the centrifuged samples. Usually, the samples must be kept at a defined temperature, e. g. at temperatures of 4° C., 22° C. or 37° C., depending on the respective application. Therefore, precautionary measures have already been taken in the past to prevent an increase in the temperature of the material to be centrifuged. On the one hand, this can be achieved by direct cooling, or by indirect cooling by means of the heat exchanger principle. In the case of indirect cooling (collateral cooling) there is no direct contact between the cooling agent and the product to be cooled and/or the envelope of the product to be cooled. Such centrifuges are described in U.S. Pat. No. 7,407,473 B2 and GB 1 018 285 A.
In the case of direct cooling, ambient air is conveyed directly at the centrifuge rotor through the centrifuge bowl with the rotor acting as a radial fan. For this purpose, the centrifuge cover and/or the centrifuge bowl include an inlet opening near a axis and an outlet opening located farther away with respect to the rotation axis. Although such a direct cooling has proved its worth, the centrifuge bowl must include an outlet opening which also allows material to escape. A disadvantage of direct cooling is the use of ambient air as a cooling agent: the sample product can at the most be cooled to the temperature of the ambient air.
In the case of indirect cooling, the rotor is enclosed in the centrifuge bowl below the centrifuge cover, and no cooling duct or the like is provided. Thus, the air circulates only inside the centrifuge bowl. Cooling is achieved with the aid of a second agent which is directed past the outside of the vessel. This agent may either be ambient air which is directed past the outside of the vessel, as is implemented in the centrifuge 5424 of Eppendorf AG, for example. Alternatively, the cooling device is composed of a compressor cooling apparatus including pipes and heat exchangers which are arranged above the equipment-side base plate, wherein, for dissipating heat, a special cooling agent is directed past the vessel via pipes which helically bear against the vessel, for example, i. e. the side walls and the bottom of the vessel. The latter variant of indirect cooling also allows for cooling the sample product to a temperature below the temperature of the ambient air. An advantage of indirect cooling is that in this process the temperature to be adjusted can be better controlled as compared with direct cooling.
In known centrifuges, the centrifuge base plate usually made of metal merely serves for passive dissipation of a portion of the heat from the inside of the housing.
But also in the case of rotors running in a vacuum in so-called ultracentrifuges this principle of passive cooling via the base plate is applied as is disclosed in DE 23 43 070 A1, for example.